Filter and fuel assembly for a light-water nuclear reactor

ABSTRACT

A filter for separating particles from cooling water in a nuclear plant of light water model. The filter has an inlet end ( 2 ) and an outlet end ( 3 ) and allow flowing through of the cooling water in a flow direction % from the inlet end to the outlet end. The filter comprises a plurality of channels ( 5 ) extending mainly in the flow from the inlet end to the outlet end. Each of the channels comprise a first ( 6 ) channel section provided closer to the inlet end, a second ( 8 ) channel section arranged closer to the outlet end, and an intermediate section ( 7 ) provided between the first and the second channel section. The intermediate section ( 7 ) has an extension d 2  in a direction x transversely to the flow direction, which is essentially larger than the extension d 2 .

FIELD OF THE INVENTION

[0001] The present invention relates to a filter for separatingparticles from the cooling water in a nuclear plant of a light-watermodel, the filter having an inlet end and an outlet end and is arrangedfor permitting flowing-through of the cooling water in a main flowdirection from the inlet end to the outlet end, the filter comprising aplurality of channels mainly extending in the flow direction from theinlet end to the outlet end, each of the channels comprising a firstchannel section provided closer to the inlet end, a second channelsection arranged closer to the outlet end an intermediate sectionprovided between the first and the second channel section. The inventionalso relates to a fuel assembly according to the preamble of the claim17.

[0002] The invention will be described in an application for nuclearplants, for purifying the cooling water flowing through a nuclear plantof light-water type, from waste material and other particles. However,the invention is not limited to any particular location of the filter inthe nuclear plant.

PRIOR ART

[0003] It is important to purify the cooling water in a nuclear plant.The purpose of the cooling water is to act as a cooling medium and as amoderator in the nuclear reactor of the nuclear plant. If waste materialor other particles are allowed to come with the cooling water in thereactor core, they may cause damages to the cladding of the fuel rods,which might lead to such damages that the nuclear fuel, i.e. uranium,will leak out into the cooling water. At larger damages, the operationof the reactor has to be shut down and the defect fuel has to bechanged. Such a change is time consuming and expensive. Waste materialand other particles may, of course, also cause damages to othercomponents in the nuclear plant, such as pumps.

[0004] Such waste material may consist of shavings, which are formed inconnection with repair of different components in the plant, metal wiresor other foreign particles which have come from outside into the plant.Particularly troublesome particles are such with an extended shape, i.e.narrow wires or shavings, which might be as small as about 10 mm long.Such particles tend to get caught further up in the fuel assembly, forexample in a spacer. The particles are vibrating in the cooling waterflow and might rub holes in the cladding of the fuel rods. At the sametime, it is also important not to filter particles considered asharmless, since everything being couth in the filter increases thepressure fall over it. Such particles may, for example, be blasting sandwith the size of 1-2 mm and mineral wool particles, which may have comeinto the cooling water by accident.

[0005] To solve this problem it is known to provide some kind of filterin the lower part of the fuel assembly, which comprises a number of fuelrods and forms the reactor core. The cooling water circulating throughthe reactor passes through this lower part of the fuel assemblies. Forexample, the base plate of the fuel assembly might be provided with anumber of small holes, through which the cooling water passes. Thus,such a filter may catch possible waste material or other particles. Twoimportant demands are put on such a filter, on one hand it must be ableto effectively catch all the particles, which might cause damages to thereactor, and on the other it shall have a low flow resistance andpressure fall. U.S. Pat. No. 5,030,412 shows such a filter for catchingwaste in the cooling water flowing through a nuclear reactor. The filterconsists of a number of parallel sheets arranged close to each otherforming passages for the cooling water. Those passages are long andnarrow and have an inlet part and an outlet part, and an intermediatebent part preventing a large portion of the particles from passingthrough the filter. Such a filter certainly prevents the particles frompenetrating further up in the fuel assembly, but it does not prevent allparticles above a certain size from continuing farther.

SUMMARY OF THE INVENTION

[0006] The object of the present invention is to provide a filter, whicheffectively filters the particles above a certain size. Further, it isdesired to provide a filter having a robust construction, which can bemanufactured in one part without the need of welding.

[0007] This object is achieved by the initially mentioned filter, whichis characterised in that the intermediate section has an extension in adirection transversely to the flow direction, which is essentiallylarger than the extension of the first and the second section channelsection in said direction, and in that the filter comprises a separatingmember arranged in the intermediate section, wherein the intermediatesection and the separating member are arranged so that at least one bentpassage for the cooling water is formed between the separating memberand a wall of the intermediate section.

[0008] Thanks to the bent passage, such a filter can effectively catchparticles in the cooling water, which particles are transported with itsextension extending essentially along the flow direction. The filter caneasily be manufactured by cutting and boring the channels directly in aplate, and thereafter inserting the separating members into thechannels. Thus, the filter can be manufactured out of one piece withoutany welded seams and thus becomes robust and durable.

[0009] According to an embodiment of the invention, the channels alsoextend in a second direction transversely to said first direction, andthe channels are arranged essentially parallel to each other. Such adesign contributes to limiting the flow resistance.

[0010] According to a further embodiment of the invention, theseparating member is shaped as a cylinder substantially extending insaid second direction transversely to the flow direction.Advantageously, the separating member extends essentially over theentire length of the channel, wherein a bent passage for the coolingwater is formed on each side of the separating member.

[0011] According to a further embodiment of the invention, theseparating member has a diameter, in a direction transversely to theflow direction, which is larger than the diameter of the first and thesecond channel section. With such a design, all particles in the coolingwater are stopped, which particles are transported with theirlongitudinal axis essentially in the flow direction and which have alength which is larger than the extension of the intermediate section inthe direction of the flow. Thus, it is possible to decide a lower limitfor the size of the particles to be caught by the filter.

[0012] According to a further embodiment of the invention, theseparating member comprises a plurality of protruding elements beingarranged at a distance from each other along the longitudinal axis ofthe separating member. Advantageously, said protruding elements areshaped as flanges surrounding the separating member. Such an arrangementwill also catch particles being transported with an extension mainlyextending transversely to the flow direction. The distance between theprotruding elements sets an upper limit for the size of the particles tobe caught by the filter.

[0013] According to a further embodiment of the invention, the distancebetween the protruding elements is essentially equal to the extension inthe flow direction of the intermediate section. The distance between theprotruding elements and the extension of the intermediate section in theflow direction decides which size of the waste is permitted to passthrough the filter. Accordingly, it is possible to design the filtersuch that it catches particles above a certain size independent of thedirection of the longitudinal axis of the particles. Also, the design ofthe filter determines in a well-defined way the size of the waste to befiltered.

[0014] According to a further embodiment of the invention, the walls ofthe intermediate section are concave. Concave walls give a soft curvedflow-path without any sharp edges, leading to a calm flow and thus a lowpressure fall. Preferably, the separating member has a bendingessentially corresponding to the bending of the walls in theintermediate section. Accordingly, a directed flow is obtained, whichdoes not give rise to any unnecessary turbulence, which in turn causespressure fall.

[0015] According to a further embodiment of the invention, the centreline of the intermediate section is essentially concentric to the centreline of the separating member. Thus, a symmetric flow is achieved onboth sides of the separating member.

[0016] According to a further embodiment of the invention, theintermediate section is essentially circular in a section parallel tothe flow direction. Advantageously, the separating member also has anessentially circular cross-section. This is advantageous from amanufacturing point of view. In another embodiment, the separatingmember may have an elliptical cross-section which is advantageous from aflow point of view.

[0017] According to a further embodiment of the invention, the firstchannel section has a flow area of, which is essentially equal to thearea of flow in the intermediate section. Thus, a good balance betweenflow resistance and catching ability is achieved.

[0018] According to a further embodiment of the invention, twoneighbouring channels have intermediate sections arranged at differentdistances from the inlet end. This is advantageous since it is possibleto locate the channels closer to each other so that more channels can bearranged on the same surface. This means that the flow resistance islimited.

[0019] According to a further embodiment of the invention, the filtercomprises a plurality of grooves arranged on the surface of the inletend and arranged essentially transversely to the longitudinal directionof the channels. Such grooves catch long and narrow waste such as wires,thanks to the fact that they lay down in the grooves. Accordingly, aportion of the waste is prevented from entering into the filter andobstructing the flow paths. Waste in the flow paths contribute toincreasing the pressure fall.

DESCRIPTION OF DRAWINGS

[0020] The present invention will now be explained by the description ofdifferent embodiment examples and with reference to the appendeddrawings.

[0021]FIG. 1 shows a top view of a filter according to the invention.

[0022]FIG. 2 shows a cross-section A-A through the filter in FIG. 1.

[0023]FIG. 3 shows a perspective view of a part of a filter according tothe invention.

[0024]FIG. 4 shows a separating member.

[0025]FIG. 5 shows a schematic side view of a fuel assembly for aboiling water reactor.

[0026]FIG. 6 shows a schematic side view of a pressure water reactor.

[0027]FIG. 7 shows a top view of a bottom part of the fuel assembly inFIG. 5.

[0028]FIG. 8 shows a cross-sectional view from the side of the bottompart in FIG. 7 along the line B-B in FIG. 5.

DESCRIPTION OF EMBODIMENTS

[0029] The FIGS. 1 and 2 show a filter 1 for separating particles fromthe cooling water in a nuclear plant. The filter 1 has an inlet end 2and an outlet end 3. The cooling water may thus flow through the filter1 from the inlet end 2 to the outlet end 3 in a main flow direction z.

[0030] The filter 1 comprises an essentially rectangular solid plate 4,in which a number of elongated spaces 5 are arranged. The plate 4 ismade of a metallic material, for example stainless steel. The spaces 5are parallelly arranged and extend along the main part of the length ofthe plate 4 in a direction y transversely to the flow direction. Thespaces 5 extend through the entire plate 4 in the flow direction z andhave openings in the inlet end 2 and the outlet end 3 so that thecooling water can flow through the filter 1. The spaces 5 definechannels, through which the cooling water has to pass before reachingthe fuel rods.

[0031] As shown in FIG. 2 the spaces 5 comprise a first section 6, anintermediate section 7, and a second section 8. The first section 6 isarranged close to the inlet end 2 and has an extension d₁ in thedirection x transversely to the flow direction. The second section 8 isarranged close to the outlet end 3 and has an extension d₃ in thedirection x transversely to the flow direction. The intermediate section7 is arranged between the first and the second section and has walls,which are concave. The walls in the first and the second section areessentially straight. Transversely to the flow direction z the space 5has in the first, the second, and the intermediate section anessentially rectangular cross-section. In a plane comprising the flowdirection z and the direction x, the intermediate section 7 has anessentially circular cross-section with a diameter d₂. In this plane,the intermediate section 7 has an inlet chord, which is equal to theextension d₁ of the first section 6. Thereafter, the space 5 widens andon its widest part, the intermediate section 7 has an extension d₂.Thereafter, the space 5 tapers and the intermediate section 7 has anoutlet chord which is equal to the extension d₃ of the second section 8.The intermediate section 7 also has a width in the direction x which islarger than the width of the first 6 and the second 8 section.

[0032] The width d₁, d₃ of the first and the second sections 6, 8 in thedirection x may be in the order 3-5 mm, for example 4 mm. The width d₂of the intermediate section 7 in the direction x may in its widest partbe in the order 7-9 mm, for example 8 mm. The length of the space 5 inthe direction y may be in the order 50-150 mm. The height of the plate4, which is the height of the space 5 in the flow direction z, may be15-30 mm, for example 20 mm.

[0033] The filter 1 further comprises a number of separating members 9in the form of elongated circular cylinders, which are arranged in themiddle of the intermediate section 7. The separating member 9 extendsalong the length of the intermediate section, in the direction y, and isarranged so that its centre axis coincides with the centre axis of theintermediate section 7, i.e. the separating member 9 is arrangedconcentric with the intermediate section 7. The separating member 9 isin its outer ends joint to the plate 4. For preventing long and narrowparticles, which has their longitudinal axis directed parallel to theflow direction z, from following the cooling water through the filter,the separating member 9 has a diameter d₄, which is somewhat larger thanthe widths d₁ and d₃ of the first 6 and the second 8 section. Thediameter d₄ of the separating member 9 may be in the order 3-5 mm, forexample 4.1 mm.

[0034] The separating member 9 divides the space in the intermediatesection 7 so that it forms two bent passages 10, 11 for the coolingwater between the first 6 and the second 8 section. The bent passages10, 11 thus have a bending in one plane, which comprises the flowdirection z and the direction x. The passages 10, 11 are formed betweenthe separating member and the walls of the intermediate section 7, whichmean that a passage is formed on each side of the separating member 9.For both passages 10, 11 applies that they have equal and in eachcross-section a constant flow area. For limiting the pressure fall, thesum of the flow areas in both passages 10, 11 is approximately equal tothe flow area in the first section. The particles accompanying thecooling water are caught in the bent passages. The height h of theintermediate section in the flow direction z determines the size of theparticles allowed to pass through the filter. Thus, all long and narrowparticles having a size larger than the height h are stopped if they aretransported parallel to the flow direction. The height h may be in theorder 5-15 mm, for example 7 mm.

[0035] For preventing long and narrow particles having theirlongitudinal axis directed transversely to the flow direction z, fromaccompanying the cooling water straight through the filter, theseparating member 9 is provided with a plurality of flanges. Thoseflanges are arranged at equal distances along the whole length of theseparating member. The flanges 12 extend from the separating member in adirection towards the walls of the intermediate section. The distance Ibetween the flanges determines the size of the particles to be caughtand may be in the order 5-15 mm, for example 7 mm. To fit in theintermediate section, the diameter of the flanges shall be a littlesmaller than or equal to the diameter d₂ of the intermediate section,i.e. in the order 7-9 mm, for example 7.9 mm.

[0036] As evident from FIG. 2, the neighbouring spaces 5 have theirintermediate sections 7 with the separating members 9 arranged displacedin relation to each other in the flow direction z, i.e. they arearranged at different distances from the inlet end 2. Accordingly, it ispossible to decease the distance between the channels and consequentlythere is room for more channels in the plate. The more space there isfor channels, the less pressure fall causes the filter.

[0037] The cooling water flowing through the spaces 5 in the filter fromits inlet end 2 to its outlet end 3 will first pass through the firstsection 6 and when it then passes through the intermediate section 7,the flow is divided into two partial flows, wherein the first partialflow passes the first bent passage 10 and the second partial flow passesthrough the second bent passage 11. Thereafter, the partial flowsconverge and the flow passes through the second channel section 8.Particles above a certain size, which are transported with their lengthaxis essentially parallel to the flow direction, are caught in the bentpassages 10, 11 and particles transported with their length axisessentially transversely to the flow direction are caught by the flanges12.

[0038] For catching a portion of the waste before it enters the filterand thus preventing pressure fall in the filter due to it being cloggedwith waste, the surface of the inlet end of the filter 1 is providedwith a number of longitudinal grooves 17 extending transversely to thelongitudinal axis of the spaces 5.

[0039]FIG. 3 shows how the spaces 5 are arranged in the solid plate 4.The channels are, for example, manufactured by boring a plurality ofcircular holes 13 from one end of the plate. The holes are bored in twodifferent levels, i.e. with two different distances from the top side ofthe plate. Every second hole is bored in the first level and every otherhole is bored in the second level. Thereafter, longitudinal slits arecut from underside of the plate 4, which slits extend through the boredholes 13 and out through the top side of the plate 4. The bored holes 13form the intermediate section 7 and the rest of the slits form the first6 and the second 8 channel section. Of course, it is also possible tofirst cut the slits and then bore the holes.

[0040]FIG. 4 shows a separating member 9 intended for being brought intothe bored holes 13 in FIG. 3. Both ends of the separating member isprovided with fixing elements 14, which are adapted so that they fix theseparating member 9 to the plate 4, for example by force fit, when theseparating member has been brought into the plate 4 through the hole 13.

[0041] The filter 1 is particularly suitable, but not exclusively, forbeing mounted in a fuel assembly for a nuclear plant. The FIGS. 5 and 6show two different types of fuel assemblies 15 and 16, which aresuitable for including the filter 1. FIG. 5 shows a fuel assembly 15intended for a boiling water reactor, BWR, which fuel assembly comprisesa top part 20 and a bottom part 21. A number of fuel rods 22 arearranged between the top part 20 and the bottom part 21. The fuel rods22 are in their lower ends joined to the bottom part 21 and it theirupper ends joined to the top part 20. The fuel assembly 15 furthercomprises spacers 23, which are apportioned along the length of the fuelrods 22 and the purpose of which is to keep the fuel rods 22 in adesired position. Further, the fuel assembly comprises a housing 24extending between the top part 20 and the bottom part 21 and includingall the fuel rods 22. A filter 1 according to the description above isarranged in the bottom part 21. A filter 1 is schematically indicated inFIG. 5. The fuel assembly 15 is adapted to allow the cooling water toflow into the fuel assembly through the bottom part 21 and in betweenthe fuel rods 22.

[0042] The bottom part 21 is closer shown in the FIGS. 7 and 8. From theFIGS. 7 and. 8 it is evident that the fuel assembly comprises fourfilters 1, which each are located in an essentially square opening 27 ofthe bottom part 21. The filters 1 are arranged parallel to each otherand the cooling water flowing into the bottom part 21 via an inletopening 28 will flow through any of the filters 1. It is further evidentfrom FIG. 8 that the intermediate sections 7 are arranged in two levels,and that two neighbouring channels have intermediate sections 7 atdifferent levels. It shall be noted that the bottom part 21 could alsocomprise one single large opening 27 having one large filter 1. Thisfilter could, for example, be machined from a solid plate and laterprovided with separating members.

[0043]FIG. 6 shows a fuel assembly 16 for a pressure water reactor, PWR.The fuel assembly 16 also comprises a top part 30, a bottom part 31, anda number of fuel rods 32. Furthermore, the fuel assembly 16 comprises anumber of guide pipes 33 extending between and joined to the bottom part31 and the top part 30. The fuel rods 32 are held by assistance ofspacers 34 which are joined to the guide pipes 33. The filter 1 is alsoin this case arranged in the bottom part 31 and is schematically drawnin FIG. 6. The cooling water which flows into the fuel assembly betweenthe fuel rods 32 will thus also flow through the filter 1. In thisembodiment, shown in FIG. 6, the fuel assembly 16 comprises only onesingle filter covering the entire area of the bottom part 31 shown inthe horizontal section, but also in this case the fuel assembly could,of course, comprise a plurality of filter 1, for example four.

[0044] The invention is not limited to the embodiments shown but can bevaried and modified within the scope of the following claims. Forexample, it is possible to manufacture such a filter in the form ofparallel profiled bars, which are joined at a chosen distance by use ofside sections, which also fixes the separating members.

1. A filter (1) for separating particles from cooling water in a nuclearplant of light-water model, the filter having an inlet end (2) and anoutlet end (3) and is adapted for permitting flowing-through of thecooling water in a main flow direction (z) from the inlet end to theoutlet end, the filter comprising a plurality of channels (5) extendingmainly in the flow direction (z) from the inlet end to the outlet end,each of the channels comprising a first (6) channel section providedcloser to the inlet end, a second (8) channel section provided closer tothe outlet end, and an intermediate (7) section provided between thefirst and the second channel section, characterised in that theintermediate (7) section has an extension (d₂) in a direction (x)transversely to the flow direction which essentially is larger than theextension (d₁, d₃) of the first (6) and the second (8) channel sectionin said direction (x), and in that the filter comprises a separatingmember (9) arranged in the intermediate section, wherein theintermediate section and the separating member are arranged so that atleast one bent passage (10, 11) for the cooling water is formed betweenthe separating member and a wall of the intermediate section.
 2. Afilter according to claim 1, characterised in that the channels (5) alsoextend in a second direction (y) transversely to said first direction(x) and transversely to the flow direction (z) and in that the channelsare arranged essentially parallel to each other.
 3. A filter accordingto claim 2, characterised in that the separating member (9) is shaped asa cylinder essentially extending in said second direction (y)transversely to the flow direction.
 4. A filter according to claim 3,characterised in that the separating member (9) extends essentially overthe entire length of the channel (5), wherein a curved passage (10, 11)for the cooling water is formed on each side of the separating member.5. A filter according to claim 3 or 4, characterised in that in saidfirst direction (x) transversely to the flow direction, the separatingmember has an extension (d₄) which is larger than the extension (d₁, d₃)of the first and the second outlet section.
 6. A filter according to anyof the claims 3-5, characterised in that the separating member (9)comprises a plurality of protruding elements (12) being arranged at adistance from each other along the longitudinal axis of the separatingmember.
 7. A filter according to claim 6, characterised in that saidprotruding elements (12) are shaped as flanges.
 8. A filter according toclaim 6 or 7, characterised in that the distance (1) between theprotruding elements is essentially equal to the extension (h) in theflow direction (z) of the intermediate section (7).
 9. A filteraccording to any of the previous claims, characterised in that the wallsof the intermediate section (7) are concave.
 10. A filter according toclaim 9, characterised in that the separating member (9) has a bendingessentially corresponding to the bending of the walls in theintermediate section (7).
 11. A filter according to any of the previousclaims, characterised in that the centre line of the intermediatesection (7) is essentially concentric to the centre line of theseparating member (9).
 12. A filter according to any of the previousclaims, characterised in that the intermediate section (7) isessentially circular in a section parallel to the flow direction.
 13. Afilter according to claim 3, characterised in that the separating member(9) has an essentially circular cross-section.
 14. A filter according toany of the previous claims, characterised in that the channels (5) havean area of flow in the first section (6) which is essentially equal tothe area of flow in the intermediate section (7).
 15. A filter accordingto any of the previous claims, characterised in that two neighbouringchannels (5) have intermediate sections (7) arranged at differentdistances from the inlet end (2).
 16. A filter according to any of theprevious claims, characterised in that it comprises a plurality ofgrooves (17) arranged on the surface of the inlet end and arrangedessentially transversely to the longitudinal direction of the channels(5).
 17. A fuel assembly (15, 16) for a nuclear plant of light-watermodel, the fuel assembly comprising a bottom part (21, 31), a top part(20, 30), and a plurality of fuel rods (22, 32) arranged next to eachother with a interspace between them, the rods being arranged betweenthe bottom part and the top part of the fuel assembly, the bottom partcomprising a filter (1) for separating particles from cooling waterbeing circulated through the fuel assembly, the filter having an inletend (2) and an outlet end (3) and is adapted for permittingflowing-through of the cooling water in a main flow direction (z) fromthe inlet end to the outlet end, the filter comprising a plurality ofchannels (5) essentially extending in the flow direction from the inletend to the outlet end, each of the channels comprising a first channelsection (6) provided closer to the inlet end, a second channel section(8) provided closer to the outlet end, and an intermediate section (7)provided between the first and the second channel section, characterisedin that in a direction (x) transversely to the flow direction, theintermediate section (7) has an extension (d₂), which essentially islarger than the extension (d₁,d₃) of the first and the second channelsection (6, 8) in said direction (x), and in that the filter comprises aseparating member (9) arranged in the intermediate section (7), whereinthe intermediate section and the separating member are arranged so thatat least one bent passage (10, 11) for the cooling water is formedbetween the separating member and a wall of the intermediate section.18. A fuel assembly according to claim 17, characterised in that thefilter and the bottom part are arranged for conducting the cooling waterinto said space.